1. Field of Endeavor
The present invention relates to the field of thermal machines, and more particularly to a steam turbine.
2. Brief Description of the Related Art
In FIG. 1, a detail of a greatly simplified view of the standard construction of a steam turbine for the high-pressure (HP) range and intermediate-pressure (IP) range is reproduced. The steam turbine 26 of FIG. 1 includes a rotor 11 which is rotatably mounted around an axis and concentrically enclosed at a distance by an inner casing 12. Between the rotor 11 and the inner casing 12 an (axial) annular, customarily bladed flow passage 13 is formed, by which the supplied steam is expanded during operation. The flow passage 13 on the inlet side is delimited in the axial direction by an especially formed-on balance piston 18 which is arranged on the rotor 11 and provided for balancing the axially acting forces. The balance piston 18 is sealed in relation to the inner casing 12 by a piston seal 19. Rings of rotor blades 15 and stator blades 17, which alternate in the flow direction, project radially into the flow passage 13. At the entry of the flow passage 13 an (encompassing) inlet scroll 14, by which the steam is guided from the outside radially inwards and deflected in a deflection region 27 in the axial direction to the inlet of the flow passage 13, is formed on the inner casing 12. The stator blades 17 are fastened on the inner casing 12 in a way which is not described in more detail. The rotor blades 15 are fastened by blade roots in each case in encompassing fastening slots 16 on the rotor 11.
At very high operating temperatures (for example in the case of ultra-supercritical (USC) steam processes and 700° C. machines), the thermal stresses in the rotor 11, and particularly in the fastening slots 16 of the rotor blades 15 there, become very large. It has therefore already been considered to provide a so-called stress-relief slot (20 in FIG. 2) in the deflection region 27 of the rotor 11 upstream of the first fastening slot 16 in order to relieve the thermal stresses in this region. It has been observed, however, that such a stress-relief slot causes high aerodynamic losses if it is located in the flow path. If the stress-relief slot on the other hand is shifted into the region of the piston seal 19 either the effect of the seal is reduced or the entire machine has to be constructed longer in the axial direction, or the stress relief of the stress-relief slot is reduced.